The present invention relates to a new bicomponent fiber, a nonwoven fabric comprising said new bicomponent fiber and sanitary articles made therefrom. The nonwoven fabric comprising the new bicomponent fiber according to the instant invention are not only excellent in softness, but also high in strength, and can be produced in commercial volumes at lower costs due to higher thoughputs and requiring less energy.
Nonwoven fabrics, such as spunbonded nonwoven fabrics and Nonwoven fabrics manufactured using carding, melt-blowing or airlaid techniques have been used in a wide variety of applications recent years, also for sanitary articles made therefrom.
A polyethylene nonwoven fabric, which resin fibers are formed of polyethylene, is known for its softness and good touch (EP-A-0,154,197). Polyethylene fibers are, however, difficult to spin, and hence difficult to allow to have a fine denier which are required for obtaining a good softness of a fabric. Nonwoven fabric formed of polyethylene fibers easily melts when subjected to heat/pressure treatment with a calender roll, and what is even worse, it easily winds itself around the roll due to low strength of the fibers. Measures have been taken against the above problems in which the treatment temperature is decreased; however, in such a case, thermal adhesion is apt to be insufficient, which leads to another problem of being unable to obtain nonwoven fabric with sufficient strength and fastness to rubbing. In actuality, a polyethylene nonwoven fabric is inferior to a polypropylene nonwoven fabric in strength.
In order to solve the above mentioned problems, there have been proposed techniques of utilizing a core-sheath-type bicomponent fiber using a resin of polypropylene, polyester, etc., as a core, and polyethylene as a sheath (Japanese Patent Laid-Open No. 2-182960 and Japanese Patent Laid-Open No. 5-263353).
However, nonwoven fabrics, which are formed of core-sheath-type bicomponent fibers, as described above, have not had both softness and strength adequate to be used as sanitary materials. Specifically, when increasing the amount of polyethylene as a constituent of sheath, the softness of the nonwoven fabric is enhanced, but its strength is not allowed to be sufficient, as a result of which it is likely to fracture during the process. On the other hand, when increasing the constituent of core, the nonwoven fabric is allowed to have sufficient strength, but is poor in softness and its quality, as a material for sanitary goods, decreases. Thus it has been difficult to obtain a nonwoven fabric having both of the above performances on a satisfactory level.
Many of these core-sheath-type bicomponent fibers comprise a polyethylene sheath with a polyester or polypropylene core. The incumbent polyethylenes typically used in such applications have recrystallization temperatures which are generally greater than 110° C.
A first solution to the aforementioned problem is disclosed in EP-A-1,057,916 which describes spunbonded nonwoven fabrics for disposable sanitary articles made from conjugated fibers. Such conjugated fibers having a high melt core and low melt sheath material can be of side-by-side type. The low melt material proposed is a polyethylene based resin having a first high melting point in the range of 120 to 135° C. and a second low melting point in the range from 90 to 125° C., the melting point of the second low melting material being at least 5° C. below the first high melting point. Such low melt polyethylene based resins are rather complex to make and cause problems during fiber spinning and later use of such conjugated fibers in the production of Nonwoven, in particular for Nonwoven fabrics manufactured using carding, melt-blowing or airlaid techniques.
However, it still would be desirable to lower the melting point of the polyethylene in order to allow faster line speeds due to lower binding temperature and lower energy usage. On the other hand, lowering the melting point of the polyethylene is associated with significant processing problems during fiber spinning. For widespread applicability for use in binder fibers, such fiber should have the following characteristics: good spinning performance, such that smoke, fiber breaks and fibers sticking together are minimized during the spinning process; the fibers also need to have a low COF to allow the ability to be texturized; good fiber tensile properties; ability to be readily cut; ability to be used in the airlaid process and ability to be bonded using the thermal air bonding process at the lowest temperature without fibers becoming sticky. Additionally, the outer layer of the bi-component fiber should have good bonding to the inner core (substrate) as well as to other fibrous products.